The present invention relates to a method and apparatus for automatically and passively adjusting the pH of the liquid used to flood the containment structure of a nuclear reactor following an accident. The liquid is recirculated in a cooling loop in order to cool the reactor core. More particularly, the present invention relates to a method and apparatus for adjusting the pH of the slightly acidic recirculating solution of borated water used to flood the containment structure of a nuclear reactor following an accident so that the borated water being recirculated becomes slightly basic.
In nuclear power plants, an emergency core cooling system is provided in order to remove the stored and fission product decay heat from the reactor core, and thus cool the reactor core, following an accident such as a pipe break resulting in the loss of the primary coolant or of the secondary coolant. Such an emergency core cooling system, for example in the case of a pressurized water reactor (PWR) plant, involves the flooding of the containment structure of the nuclear reactor, e.g. to a level of about 10 feet, with a slightly acidic solution of borated water which is then recirculated through the legs of the reactor coolant system. The emergency core cooling system, which includes pumps, residual heat removal heat exchangers, etc., recirculates the water by pumping it out of an emergency containment sump provided in the floor of the containment structure, which is conventionally formed substantially of concrete and which will leach chlorides into the water being recirculated. In order to protect stainless steel components of the nuclear reactor plant, i.e. in the reactor, the coolant piping, and the safeguard systems, against chloride stress corrosion and cracking, the pH of the water being recirculated must be adjusted to greater than or equal to pH=8.5 within 48 hours after the accident.
In the known nuclear reactor plants, the above mentioned pH adjustment is normally carried out by the addition of a caustic solution of sodium hydroxide to a spray system provided within the containment structure which sprays the caustic solution throughout the atmosphere of the containment structure. This known method of adjusting the pH has two significant disadvantages. Firstly, for some accident scenarios, i.e., depending on the type and severity of the accident, the spray system may not be activated. Secondly, it may not be desirable for the caustic solution of sodium hydroxide to be sprayed over the various components of the containment structure and other plant components for reasons of clean up and the caustic effects on paint, insulation, etc.